When a pit-type lifting jack is used in maintenance operation of non-disintegrated rack car of motor train unit and urban rail vehicles, it is necessary to lift the vehicle to a certain height with the supporting wheel of bogie lifting unit, and then support four lifting points of track car using four supporting heads of the pit-type lifting jack, so as to replace the vehicle bogie and carry out the maintenance and replacement operations for the electrical facilities at the lower part and the top of vehicle.
With the extensive operation of motor train units in China, there are more and more cities that have started the urban rail transportation. In order to ensure the operation safety and reliability of above vehicles, it is necessary to carry out regular repair and maintenance on the wheel sets of the vehicle bogie and underbody. The pit-type lifting jack plays an important role in the vehicle repair and maintenance process. Currently, there are two kinds of structure of supporting heads of pit-type lifting jack; one is the supporting head with weighing function, and the other is the basic type supporting head. The basic type supporting head has a simple structure, and it is inconvenient to adjust a variety of supporting heads simultaneously. At present, most supporting heads used are the traditional supporting heads with the weighing function, as shown in FIG. 1 and FIG. 2. Chinese patent (Publication No. CN102060027A) discloses a detection type supporting head of lifting system with weighing function. This kind of supporting head comprises a supporting head body and a load-bearing detection device arranged on the supporting head body. By adjusting the lifting positions of the mandril, the distance between the load-bearing plate and the baffle can be adjusted. It can achieve adjustment conveniently and display the weight of the whole vehicle, but when adjusting a variety of supporting heads, it is necessary to adjust them one by one repeatedly, with poor precision and heavy workload. During lifting maintenance operation, the size error of four steel plates welded on the side beam of vehicles can not be avoided, with a certain height difference between the four steel plates, thus, they can not be on the same plane. For the above supporting heads with weighing functions and basic type supporting heads, it is inconvenient to adjust a variety of supporting heads simultaneously, and when the load-bearing surface of the existing supporting head and the lifting column are an integral structure, the upper surface of supporting head can not form a state of line contact or point contact with four steel plates on the side beam completely when lifting, which easily causes damage to the vehicle.